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LOS ANGELES (CBS/AP) — Those envisioning July 4 celebrations at the beach may be swimming at their own risk.

A new study has found that the number of beach closures nationwide due to dirty water soared last year.

The Natural Resources Defense Council report released Wednesday found that beach closures and advisories increased by 29 percent in 2010 compared to a year earlier.

Eleven percent of California’s beaches reported elevated levels of bacterial contamination, the largest amount in five years. Among those at the top of the list were Avalon Beach, Cabrillo Beach and Colorado Lagoon in Los Angeles County, Poche County Beach and Doheny State Beach in Orange County and Candlestick Point in San Francisco County.

Swimming in such pollution can cause gastrointestinal, respiratory and other illnesses.

The NRDC awarded superstar status to four beaches, none of which are in California: Rehoboth Beach-Rehoboth Avenue Beach and Dewey Beach in Delaware, Park Point Lafayette Community Club Beach in Minnesota, and Hampton Beach State Park in New Hampshire. These beaches received the special rating for having perfect test ratings the past three years.